From the discovery of superconductivity in 1911 to the recent past, essentially all known superconducting materials were elemental metals (e.g., Hg, the first known superconductor) or metal alloys or intermetallic compounds (e.g., Nb.sub.3 Ge, probably the material with the highest transition temperature T.sub.c known prior to 1986).
Recently, superconductivity was discovered in a new class of materials, namely, metal oxides. See, for instance, J. G. Bednorz and K. A. Muller, Zeitschr. f. Physik B--Condensed Matter, Vol. 64, 189 (1986), which reports superconductivity in lanthanum barium copper oxide.
The above report stimulated worldwide research activity, which very quickly resulted in further significant progress. The progress has resulted, inter alia, to date in the discovery that compositions in the Y-Ba-Cu-O system can have superconductive transition temperatures T.sub.c above 77 K., the boiling temperature of liquid N.sub.2 (see, for instance, M. K. Wu et al, Physical Review Letters, Vol. 58, Mar. 2, 1987, page 908; and P. H. Hor et al, ibid, page 911).Furthermore, it has resulted in the identification of the material phase that is responsible for the observed high temperature superconductivity, and in the discovery of compositions and processing techniques that result in the formation of bulk samples of material that can be substantially single phase material and can have T.sub.c above 90 K. (see, for instance, R. J. Cava et al, Physical Review Letters, Vol. 58(16), pp. 1676-1679), and D. W. Murphy et al, Physical Review Letters, Vol. 58(18), pp. 1888-1890.
The excitement in the scientific and technical community that was created by the recent advances in superconductivity is at least in part due to the potentially immense technological impact of the availability of materials that are superconducting at temperatures that do not require refrigeration with expensive liquid He. Liquid nitrogen is generally considered to be one of the most advantageous cryogenic refrigerants, and attainment of superconductivity at liquid nitrogen temperature was a long-sought goal which until very recently appeared almost unreachable.
A huge volume of work on high temperature superconductors has been reported since publication of the above seminal papers. Most of the work deals with YBa.sub.2 Cu.sub.3 O.sub.x (the so-called 1-2-3 compound) and related compounds. In all of these compounds the superconducting phase is perovskite-like, typically having orthorhombic crystal structure, and the compounds that exhibit high (ie., T.sub.c .gtoreq.77 K.) temperature superconductivity generally contain one or more rare earth elements.
The discovery of high T.sub.c superconductivity in some mixed copper oxides also stimulated a search for compounds exhibiting still higher T.sub.c. Despite numerous reports of observation of T.sub.c above 100 K. (even above room temperature) in 1-2-3 and related compounds, up until recently no stable superconductors with T.sub.c higher that YBa.sub.2 Cu.sub.3 O.sub.7 have been reported. Thus wire service and newspaper reports that groups in the USA and in Japan have discovered stable high T.sub.c superconductivity in samples containing Bi, Al, Sr, Ca, Cu, and oxygen, and Bi, Sr, Ca, Cu, and oxygen, respectively, were received with considerable interest. See, for instance, New York Times, Jan. 26, 1988.
It soon became apparent that the Bi-Sr-Ca-Cu oxide samples were multiphase material and frequently contained three superconducting phases, having T.sub.c near 120, 105, and 80 K. respectively. Although the existence of these phases was recognized, their compositions and structures remained unknown.
The importance of having available a superconductor with still higher T.sub.c than the 1-2-3 compound is probably evident to everyone skilled in the art. Furthermore, the newly discovered materials contain only relatively common and inexpensive elements. Thus there is strong economic interest in the development of these materials. Finally, the prior art high T.sub.c superconductors have properties that are obstacles to their technological use. For instance, the prior art materials are relatively brittle, both in single crystal form and in the form of sintered bodies.
For general overview of some potential applications of superconductors see, for instance, B. B. Schwartz and S. Foner, editors, Superconductor Applications: SQUIDS and MACHINES, Plenum Press 1977; and S. Foner and B. B. Schwartz, editors, Superconductor Material Science, Metallurgy, Fabrications, and Applications, Plenum Press 1981. Among the applications are power transmission lines, rotating machinery, and superconductive magnets for e.g., fusion generators, MHD generators, particle accelerators, levitated vehicles, magnetic separation, and energy storage, as well as junction devices and detectors. It is expected that many of the above and other applications of superconductivity would materially benefit if high T.sub.c superconductive material could be used instead of the previously considered relatively low T.sub.c materials. In particular, it is expected that superconductors according to the invention can advantageously be used in these and/or other applications, in a manner similar to that proposed for the prior art high T.sub.c superconductors.